1. Field of the Invention
The present invention relates to an improved process for preparing polyimides. More specifically, the present invention enables one to prepare polyimides of high molecular weight from dianhydrides and relatively unreactive diamines.
2. Description of the Background Art
Dupont and Bilow have disclosed polyimide composition coatings useful to encapsulate solar cells in their co-pending commonly-assigned, application Ser. No. 693,024 (now U.S. Pat. No. 4,592,925, issued June 3, 1986) and application Ser. No. 693,027, both filed Jan. 18, 1985 which are both divisional applications of Ser. No. 451,137 (now abandoned). These applications are expressly incorporated herein by reference.
The polyimide coatings of the above DuPont and Bilow patent application are formed from a polyimide composition which has the recurring structural unit shown below: ##STR3## where R is: ##STR4## and n has a contemplated value range from 10 to about 2,000, preferably, from about 10 to about 1000.
The polyimide which is most preferred according to the DuPont and Bilow invention is the meta amino phenylene derivative of Formula II above, and having the recurring structural unit: ##STR5## and its precursor has the polyamic acid structure: ##STR6## where n has the value noted above.
The polyimide of Formula I above has been prepared by the reaction of substantially equal molar proportions of the two monomers 2,2-bis(3- or 4-aminophenyl) hexafluoropropane and 4,4'-hexafluoroisopropylidene [bis(phthalic anhydride)], in a solvent for such monomers. The solvents which have been disclosed for use include, for example, dimethylsulfoxide, tetrahydrofuran, N-methyl pyrrolidinone, N-methylformamide, dimethylformamide and N,N-dimethylacetamide and mixtures thereof. The resulting polyamic acid solution has been cast as a film and the film imidized to the polyimide Structure I above. Both the polyamic acid and the polyimide have been characterized as having an inherent viscosity of at least 0.1, usually 0.3-0.5. The inherent viscosity of the polyimide is measured at 30.degree. C. as a 0.5% solution in a suitable solvent, such as cold concentrated (95%) sulfuric acid or methanesulfonic acid.
In preparing the coated solar cells, a solution of the polyamic acid precursor-of Formula V above in a solvent, such as tetrahydrofuran, at a concentration of about 10 to about 30% of the polyamic acid, has been used as a varnish for application to the active surface of a solar cell.
After application of the polyamic acid varnish to the solar cell, over a primer where desired, the solvent is essentially evaporated off and the amic acid polymer is converted into the imidized or polyimide structure of Formula IV by heating such amic acid polymer at about 250.degree. C. Lower temperatures, such as at 120.degree. C., can also be used to promote the imidization, but the reaction rate is slower and the elimination of solvent residues is slower. Preferred imidization temperatures range between about 160.degree. C. and 250.degree. C.
The DuPont and Bilow polyimide provides a coating which (1) is colorless, (2) is transparent to the solar radiation in the visible light spectrum, (3) is relatively non-brittle, (4) has a high degree of thermal stability, (5) readily transmits solar radiation without appreciable degradation, (6) is heat resistant, (7) does not degrade significantly when exposed to ultraviolet radiation, and (8) is highly effective in protecting against electrons and low energy proton radiation.
However, the DuPont and Bilow composition, due to its relatively low molecular weight as obtained, cannot be used as a manipulatable, free-standing film but must be applied in the form of a coating. While the DuPont and Bilow composition is satisfactory for its intended purpose, there exists in the art a need for such polyimide materials which can be provided in the form of a free-standing film or other useful form.